Prevent deadlock in arc_read in Linux memory reclaim callback

Using zfs with Lustre, an arc_read can trigger kernel memory allocation
that in turn leads to a memory reclaim callback and a deadlock within a
single zfs process. This change uses spl_fstrans_mark and
spl_trans_unmark to prevent the reclaim attempt and the deadlock
(https://zfsonlinux.topicbox.com/groups/zfs-devel/T4db2c705ec1804ba).
The stack trace observed is:

     #0 [ffffc9002b98adc8] __schedule at ffffffff81610f2e
     openzfs#1 [ffffc9002b98ae68] schedule at ffffffff81611558
     openzfs#2 [ffffc9002b98ae70] schedule_preempt_disabled at ffffffff8161184a
     openzfs#3 [ffffc9002b98ae78] __mutex_lock at ffffffff816131e8
     openzfs#4 [ffffc9002b98af18] arc_buf_destroy at ffffffffa0bf37d7 [zfs]
     openzfs#5 [ffffc9002b98af48] dbuf_destroy at ffffffffa0bfa6fe [zfs]
     openzfs#6 [ffffc9002b98af88] dbuf_evict_one at ffffffffa0bfaa96 [zfs]
     openzfs#7 [ffffc9002b98afa0] dbuf_rele_and_unlock at ffffffffa0bfa561 [zfs]
     openzfs#8 [ffffc9002b98b050] dbuf_rele_and_unlock at ffffffffa0bfa32b [zfs]
     openzfs#9 [ffffc9002b98b100] osd_object_delete at ffffffffa0b64ecc [osd_zfs]
    openzfs#10 [ffffc9002b98b118] lu_object_free at ffffffffa06d6a74 [obdclass]
    openzfs#11 [ffffc9002b98b178] lu_site_purge_objects at ffffffffa06d7fc1 [obdclass]
    openzfs#12 [ffffc9002b98b220] lu_cache_shrink_scan at ffffffffa06d81b8 [obdclass]
    openzfs#13 [ffffc9002b98b278] shrink_slab at ffffffff811ca9d8
    openzfs#14 [ffffc9002b98b338] shrink_node at ffffffff811cfd94
    openzfs#15 [ffffc9002b98b3b8] do_try_to_free_pages at ffffffff811cfe63
    openzfs#16 [ffffc9002b98b408] try_to_free_pages at ffffffff811d01c4
    openzfs#17 [ffffc9002b98b488] __alloc_pages_slowpath at ffffffff811be7f2
    openzfs#18 [ffffc9002b98b580] __alloc_pages_nodemask at ffffffff811bf3ed
    openzfs#19 [ffffc9002b98b5e0] new_slab at ffffffff81226304
    openzfs#20 [ffffc9002b98b638] ___slab_alloc at ffffffff812272ab
    openzfs#21 [ffffc9002b98b6f8] __slab_alloc at ffffffff8122740c
    openzfs#22 [ffffc9002b98b708] kmem_cache_alloc at ffffffff81227578
    openzfs#23 [ffffc9002b98b740] spl_kmem_cache_alloc at ffffffffa048a1fd [spl]
    openzfs#24 [ffffc9002b98b780] arc_buf_alloc_impl at ffffffffa0befba2 [zfs]
    openzfs#25 [ffffc9002b98b7b0] arc_read at ffffffffa0bf0924 [zfs]
    openzfs#26 [ffffc9002b98b858] dbuf_read at ffffffffa0bf9083 [zfs]
    openzfs#27 [ffffc9002b98b900] dmu_buf_hold_by_dnode at ffffffffa0c04869 [zfs]

Signed-off-by: Mark Roper <markroper@gmail.com>

module/zfs/arc.c

@@ -5525,6 +5525,7 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 	arc_buf_hdr_t *hdr = NULL;
 	kmutex_t *hash_lock = NULL;
 	zio_t *rzio;
+	fstrans_cookie_t cookie = spl_fstrans_mark();
 	uint64_t guid = spa_load_guid(spa);
 	boolean_t compressed_read = (zio_flags & ZIO_FLAG_RAW_COMPRESS) != 0;
 	boolean_t encrypted_read = BP_IS_ENCRYPTED(bp) &&
@@ -5997,6 +5998,7 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 	/* embedded bps don't actually go to disk */
 	if (!embedded_bp)
 		spa_read_history_add(spa, zb, *arc_flags);
+	spl_fstrans_unmark(cookie);
 	return (rc);
 }